If you want to use an audio source within OSX (iTunes, Spotify, Soundcloud, or even DJ software such as Serato) there are a few additional steps.

First install ​​​Soundflower​​​, an app that let’s one route audio outputs to inputs, like a software patch cable. This lets you pipe in your system audio to the audio input. The problem is in doing so it will divert your output from your speakers and effectively mute the audio.

Using the built-in Apple utility ​​​Audio MIDI Setup​​​ you can create a Multi-Output Device, which is effectively a software Y-Splitter. Now you can route your output to Soundflower/Audio-Input ​​AND​​ your speakers.